Conventional internal combustion engines use a camshaft and associated linkages to open and close intake and exhaust valves during engine operation. As such, the valve timing is determined during design and manufacturing and remains fixed throughout the life of the engine, neglecting changes due to wear. Determination of the valve timing requires a compromise between engine performance, fuel economy, and emissions based on a typical application for a particular engine model. As such, it is desirable to vary the intake and/or exhaust valve actuation timing based on current engine operating parameters to optimize engine performance, fuel economy, and emissions. In addition, variable valve timing may be used to provide an engine braking function.
A number of approaches have been used to increase the control authority over operation of engine intake and exhaust valves. While hydraulic assisted/controlled valve actuators provide some benefits associated with variable valve timing, electronic or electromagnetic actuators are more versatile for a variety of applications. Electromagnetic valve actuators allow direct electronic control of the engine valves. In addition to controlling the timing of the opening and closing of the valve, the valve displacement can be varied in accordance with current engine operating conditions.
A variety of design and implementation challenges must be overcome to provide a commercially viable electromagnetic valve actuator. Energy efficiency of the actuator should be considered so that the benefits of variable valve timing are not defeated by additional power requirements of the actuator as compared to a mechanical or hydromechanical system. In addition, the actuator should be capable of providing a sufficient force to accelerate the valve with a relatively high peak acceleration (3500 m/sec.sup.2 for example) while controlling the valve closing velocity to a small value(preferably less than 1 m/sec).